Measurement-based model of a wide-bore CT scanner for Monte Carlo dosimetric calculations with GMCTdospp software

Technical Note: Measurement of bow tie profiles in CT scanners using radiochromic film

PURPOSE

To provide a noninvasive technique to measure the intensity profile of the fan beam in a computed tomography (CT) scanner that is cost effective and easily implemented without the need to access proprietary scanner information or service modes.

METHODS

The fabrication of an inexpensive aperture is described, which is used to expose radiochromic film in a rotating CT gantry. A series of exposures is made, each of which is digitized on a personal computer document scanner, and the resulting data set is analyzed to produce a self-consistent calibration of relative radiation exposure. The bow tie profiles were analyzed to determine the precision of the process and were compared to two other measurement techniques, direct measurements from CT gantry detectors and a dynamic dosimeter.

RESULTS

The radiochromic film method presented here can measure radiation exposures with a precision of ∼ 6% root-mean-square relative error. The intensity profiles have a maximum 25% root-mean-square relative error compared with existing techniques.

CONCLUSIONS

The proposed radiochromic film method for measuring bow tie profiles is an inexpensive (∼$100 USD + film costs), noninvasive method to measure the fan beam intensity profile in CT scanners.

GMctdospp: Description and validation of a CT dose calculation system

PURPOSE

To develop a Monte Carlo (MC)-based computed tomography (CT) dose estimation method with a graphical user interface with options to define almost arbitrary simulation scenarios, to make calculations sufficiently fast for comfortable handling, and to make the software free of charge for general availability to the scientific community.

METHODS

A framework called GMctdospp was developed to calculate phantom and patient doses with the MC method based on the EGSnrc system. A CT scanner was modeled for testing and was adapted to half-value layer, beam-shaping filter, z-profile, and tube-current modulation (TCM). To validate the implemented variance reduction techniques, depth-dose and cross-profile calculations of a static beam were compared against DOSXYZnrc/EGSnrc. Measurements for beam energies of 80 and 120 kVp at several positions of a CT dose-index (CTDI) standard phantom were compared against calculations of the created CT model. Finally, the efficiency of the adapted code was benchmarked against EGSnrc defaults.

RESULTS

The CT scanner could be modeled accurately. The developed TCM scheme was confirmed by the dose measurement. A comparison of calculations to DOSXYZnrc showed no systematic differences. Measurements in a CTDI phantom could be reproduced within 2% average, with a maximal difference of about 6%. Efficiency improvements of about six orders of magnitude were observed for larger organ structures of a chest-examination protocol in a voxelized phantom. In these cases, simulations took 25 s to achieve a statistical uncertainty of ∼0.5%.

CONCLUSIONS

A fast dose-calculation system for phantoms and patients in a CT examination was developed, successfully validated, and benchmarked. Influences of scan protocols, protection method, and other issues can be easily examined with the developed framework.

Effect of ROI filtering in 3D cone-beam rotational angiography on organ dose and effective dose in cerebral investigations

The assessment of intracranial aneurysms is increasingly performed using three-dimensional cone-beam rotational angiography (3D CBRA). To reduce the dose to the patient during 3D CBRA procedures, filtered region-of-interest imaging (FROI) is presented in literature to be an effective technique as the dose in regions of low interest is reduced, while high image quality is preserved in the ROI. The purpose of this study was to quantify the benefit of FROI imaging during a typical 3D CBRA procedure in a patient's head region. A cone-beam rotational angiography unit (Infinix) was modeled in GMctdospp, an EGSnrc-based Monte Carlo software, which calculates patient dose distributions in rotational computed tomography. Kodak Lanex, a gadolinium compound, was chosen to be the ROI filter material. The adult female ICRP reference phantom was integrated in GMctdospp to calculate organ and effective doses in simulations of FROI-CBRA examinations. During the Monte Carlo simulations, different parameters as the ROI filter thickness, the ROI opening size, the tube voltage, and the isocenter position were varied. The results showed that the reduction in dose clearly depends on these parameters. Comparing the reduction in organ dose in standard 3D CBRA and FROI-CBRA, a maximum reduction of about 60%-80% could be achieved with a small sized ROI filter and about 40%-70% of the dose could be saved using a ROI filter with a large opening. Further we could show that dose reduction strongly depends on filter thickness, the location of the organ in the radiated area, and the position of the isocenter. As a consequence, dose reduction partially differs from theoretically calculated values by a factor up to 1.6. The effective dose could be reduced to a minimum of about 40%. Due to the fact that standard 3D CBRA is only used for the assessment of aneurysms at present and, thus, most of the patient dose originates from the aneurysm treatment (with 2D techniques) itself, the dose reduction effect of ROI filtering in 3D CBRA tends to be much smaller, if the patient dose of a whole aneurysm treatment procedure is considered.